1. Field of the Invention
The present invention relates to a photodetector of a liquid crystal display apparatus, and more particularly to a luminance control device using the same. More particularly, the present invention relates to a photodetector that detects the quantity of light from the backlight of a liquid crystal display apparatus, and a luminance control device using this photodetector for controlling luminance of the backlight.
2. Description of the Background Art
In the last few years, liquid crystal displays have become widely used in personal computers, word processors and the like. A light source called a backlight is provided in the liquid crystal display to illuminate the surface plane from the rear. A cold cathode fluorescent tube, for example, is used as the light source. The backlight can have the brightness adjusted arbitrary by a user. However, the contrast may be degraded due to change in the brightness of the ambient light.
A display device for maintaining an appropriate display state is disclosed in Japanese Patent Laying-Open No. 60-26990, for example. According to this display device, the light of a light source is directly detected by a light sensor. Also, the provision of a light blocking film to improve the detection performance of only the light source is disclosed.
Japanese Patent Laying-Open No. 62-125329 discloses a transmissive type liquid crystal display including a back photodetector device for detecting indirectly the light of a lamp through a transparent plate. A light blocking layer is formed around the detector device to block off ambient light.
In general, liquid crystal display units are commercially available from manufacturers in the form of a unitary element in which a liquid crystal panel, a backlight, a light directing plate, and a reflector sheet are provided integrally. The provision of a light sensor therein will require a particular modification, which results in an increase in cost. In the case where the light sensor is to be provided between the liquid crystal panel and the backlight, the light sensor must be located at a corner location that does not interfere with the liquid crystal panel. There is a disadvantage in that the area it occupies is increased and the accuracy with which it detects by is degraded.
The quantity of light from a backlight is reduced over long periods of usage. The end of the backlight is set as the time when the luminance has become lower than 50% of the initial value. Since it is difficult for a general user to determine this value, a detecting device that notifies a user is disclosed in, for example, Japanese Patent Laying-Open No. 5-13178. According to this device, the quantity of the light from the backlight is detected by a light quantity sensor and amplified. An alarm is sounded to notify the user when the level becomes lower than 50% of its initial value.
A cold cathode fluorescent tube used as the backlight shows a variation in brightness as an initial characteristic. Light is not emitted continuously at the same luminance. As a result of this characteristic, it is difficult to accurately detect when the luminance drops below 50% for each of the backlights. In general, a phototransistor, a photodiode, and the like are used as the quantity light sensor. As the quantity light sensor per se has some deviation, a reduction of 50% in luminance is difficult to be accurately detected.